Picture storage tube



Oct. 14,1958

M. KNo| PICTURE STORAGE TUBE a sheeTsf-sheet 1 Filed lJune 26, 1952IN1/Tyrol? X Knall Get. l14, 1958 M. KNoLL 2,856,559

` PICTURE STORAGE TUBE Filed' June 2e, 1952 2 sheets-sheet 2 UnitedStates atent PICTURE STORAGE TUBE Knoll, Princeton, N. J., assignor toRadio Corporation of America, a corporation of Delaware Application June26, 1952, Serial No. `295,768

18 Claims. (Cl. 315-12) This vinvention is directed to a viewing tube,and more particularly to a charge storage device having a fluorescentscreen to provide a visual display of information.

This application is a continuation-impart of the copending applicationSerial No. 254,999, filed November v6, 1951, now abandoned.

There is a class of electron discharge tubes which utilize a fine meshscreen upon which can be established a pattern of charges correspondingto information to be reproduced for visual inspection. Within the tube asource of electron emission provides an electron discharge directedthrough the storage screen. On the opposite side of the screen there ismounted a fluorescent screen to receive the electron discharge and tovprovide a visual display. The storage screen controls the amount of theelectron discharge passing through it from point to point, so that theluminescent display provided by the uorescent screen is varied frompoint to point, in accordance with the charge pattern on the storagescreen. The pattern on the storage screen may be established by either aphotocathode providing an electron image discharge or by an electrongun, whose beam is modulated in accordance with incoming signals, whileit is being scanned over the surface of the storage screen. Such storageviewing tubes provide a greater intensified visual display than .ispossible with asingle modulated electron beam.

However, it has been diicult to provide satisfactory visual displays onthe fluorescent screen of Vsuch tubes described above. The electronspenetrating through the storage screen tend to spread and diffuse to apoint that the visual display or picture los-es resolution.

Itis therefore an object of my invention to provide an improved chargecontrol storage tube.

It is a further object of my invention to provide a charge controlstorage tube in which the resolution of the visualgpicture `is greatlyimproved.

In particular the invention is directed to a charge controlled storagetube with an electron optical `system consisting of a critical spacingof a tine mesh screenfrom the uorescent screen of the tube, whereby theelectrons penetrating through the screen are brought to a well .focuseddot raster on the fluorescent screen. This provides 'an optimumresolution of the resulting picture. The conditions for providing anelectron lens raster is preferably that in which a high potential eldgradient exists between the uorescent screen and the storage grid and arelatively low potential eld, as compared with 4the fOrmeriieId, ismaintained on the vother side of the storage grid. Y

Fig. 1 is a sectional view of a 'charge controlled storage tubeinaccordance vwith my invention.

Figs. A2 and g3 are graphical representations of conditions 'existingduring operation of the tube of Figure l.

VFigure 4.s a sectional Aview of an image tube using a chargelcontrolled storage screen, .in .accordance with my invention.

Figures 5 and 6 are graphical illustrations of the secondary emissioncharacteristics of magnesium 'uoi'ide and silica respectively.

Fig. 1 discloses a charge control storage tube consisting of anevacuated envelope 10 having two neck portions respectively indicated as12 and 14. Within the envelope neck 12 is an electron gun 16 which mayb'e considered as the viewing gun, and which may also be used forerasing. Within the neck 14 is a second electron gun structure 18 forproviding a modulated beam of electrons which is accelerated down theneck 14 into the envelope portion 10. Gun 18 is considered as thewriting gun but may lalso be used for erasing. Mounted at the large endof the envelope portion is a target assembly 20 consisting of (a) jaglass support sheet 22 having, on its surfacefacing the electron guns 16and 18, a ythin lm lof transparent conductive material 24, which may beformed of metal or a metallic compound such as tin oxide for example. Ontop of conductive film 24, is formed a uorescent screen 26 which is of aphosphor material which willuoresce under electron bombardment. Closelyspaced from the surface of iluorescent scr-een 26, and at a distance inthe order of several millimeters, is (b) la ne metal mesh storage andfocusing screen 28, and (c) a second metal mesh screen 30 is' spaced`beyond screen 28 at a distance in the order of 10 mm. Screen 30 Vmay beof a woven stainless steel mesh of 'neness in the order of 230 mesh perinch, while screen 28 may be a metal screen having a fineness in theorder of 200 mesh per inch. Storage screen 28 is provided on lthe gunside with a coating 32 of dielectric insulating material, such as silicaor magnesium uoride, and having a thickness in the order of severalmicrons'.

Screen 20 is mounted on a ring 21 of insulating ma'- terial fixed withinthe tube envelope 10, adjacent to the large opening or face plate 11ofthe tube. Fixed to the insulating mounting ring 21 is an annularsupport ring 23, supporting, intermediate itsl ends, the glass targetsheet 22. The fine metal mesh screen `28 is iixed across the open end ofthe metal vsupportring 23 which faces the electron guns `16 and 18.Screen 28 may be fastened to support ring 23 in any manner such as bywelding, for example. Also, mounted on the insulating mounting n'ng 21is a second annular metal support ring 25 across the open end of whichis mounted a metal mesh screen 30, as shown in Figure v1. TheAconductive target film 24 of the uorescent screen is insulated from themetal support ring 23 by the glass support sheet 22, as show-ri inFigure l. The conductive film .24 is connected asis shown by a lead 27to a source of potential outside the tube envelope 10. Furthermore, themetal mesh screen 28 is provided with a potential during tube operationby connecting alead 29 to the metal support ring 23, to which metal mesh28 is attached. In a similar manner, metal mesh 30 is `connected to asource of `potential by a lead 31 attached to metal support ring 25.Normally the electrodes 30 and v24 are maintained during tube operationat a constant potential. However, the metal mesh electrode 28, which isthe storage screen, is connected by a variable .adjustment to a voltagedivider 33 so that the voltage of screen 28 may be varied from a voltagesomewhat more negative than 20 volts, relative to ground, to a potentialin the order of 1,000 volts positive relative to ground.

Electron beam 38 is :scanned 'over the surface ofthe storage screenelectrode 28 in any well known manner, as for example, by two .pairs ofelectrostatic deilection plates 40 and 42, respectively. As -is wellknown, the pairs of plates 40 and 42 may be each connected to sources ofsaw-tooth voltages for providing vline and :frame scanning of the beamover the surface of the mesh screen 28.

The viewing gun 16 consists of a cathode electrode 7 having an electronemitting'coating on its surface facing screen 20. A control electrode13, a rst accelerating electrode 15, and a second accelerating electrode17 are mounted successively along the axis of gun 16 and between cathode7 and screen 20. These electrodes are maintained, during tube operation,at appropriate voltages to form the electron emission from cathode 7into a wide beam or spray 44 of electrons. The inner surface of theenvelope 10 is coated by a conductive coating 34 of colloidal graphiteadjacent to the electron gun 16. Coating 34 may be maintained at thesame potential as the second accelerating electrode 17 of gun 16. Asecond wall coating 36 extends from a point spaced from but adjacent tocoating 34 over the bulb wall enclosing the target assembly 20.` Thiscoating 36 is operated at a different potential than that of coating 34and in this mannerprovides a collimating electron lens between the twocoat- `ings to align the electrons of the spray beam 44, so that theyare directed on to the target 20 substantially normal thereto.

Electron gun 18, in like manner, consists of a cathode electrode 19 anda control grid electrode 35 enclosing cathode 19. Spaced alongrthe gunaxis toward the target are in order,1a iirst accelerating electrode 37and a second accelerating electrode 39. The wall coating 34 extends intothe neck 14 of the writing gun and forms a third accelerating electrodefor focussing the electrons of gun 18 as a beam 38 onto the target '20.Incoming signals may be applied to the control grid 35 by anyappropriate circuit means 43.

The voltages of the several electrodes disclosed in Figure 1 areillustrative of voltage values which can be used during operation of thetube of the type described. However, these voltages are not limiting.For example, mesh grid may be operated between 200vvolts and 2,000 voltspositive relative to ground. The conductive coating 24 may be operatedwithin a range of 2,000 volts to'20,000 volts positive relative toground, while storage and focusingscreen 28 may be varied between aminus 100 volts relative to ground to 2,000 volts positive to ground.

As shown in Figure 1, the metal mesh grid 30 is operated at around 1,000volts positive with respect to ground or with respect to the cathodepotential of the viewing gun 16. Also, the conductive coating 24 of thefluorescent screen is operated at close to 8,000 volts positively withrespect to ground or viewing gun cathode potential. The potential of themetal mesh 28 supporting the insulating coating 32` is operated, in thetubeV described and shown in Figure 1, within a voltage range varyingfrom a negative voltage in the order of 20 volts with respect to groundto a voltage in the order of 1,000 volts positive with respect toground.

In accordance with the invention, the tube of Figure l may be operatedas follows:

To prepare the target for storing a charge on the insulating coating 32,it is necessary to establish a` uniform potential over all of thesurface of ilm 32. This is done by turning on the viewing gun 16. Theelectrons of beam 44 are accelerated withenergies up to 1,000 voltsthrough the screen 30. The potential of screen 28 is set with.adjustable lead 29 to a sufficiently positive potential that theelectrons of beam 44 will strikethe insulating surface of 32 atvelocities or energies to initiate a secondary electron emission fromall portions of film 32. If screen 28 is set below the rst cross-overpotential, or above the second cross-over potential, of thesecondaryemission curve of the insulating material forming lm 32, then.the surfaceof film 32 will be driven toa uniform potential approachingviewing gun cathode potential. lf the potential of screen 32 is setbetween the rst and second cross-over points of the secondary emissioncurve of the material of film 32, the surface 32 will be driven posi-Itively to a potential approaching that of collector screen 30.

In the tube described and shown in Figure 1, film 32 may be silica andelectrons striking a silica llm of this type with energies above 50Volts will initiate a secondary emission greater than unity or in whichthe number of secondary electrons leaving any point of the dielectricsurface 32 is greater than the number of electrons of beam 44 strikingthat point. Depending upon the potential at which screen 28 is set,electrons of beam 44 will drive the insulating surface 32 to a uniformpotential. A fraction of a second only is required to drive all of thesurface to a uniform potential.

The viewing gun is then switched off and the focusing and storage screen28 is set at a voltage in the neighborhood of 20 volts negative withrespect to the viewing gun cathode potential, or ground. The viewing gunis then turned on again. However, the surface of the insulating lm 32,because of its thinness, is closely coupled by capacity to the metalscreen 28 and assumes substantially the same `potential as the screen28. The electrons of beam 44, which are accelerated through the positivecollector screen 30 now enter a retarding field adjacent the surface offilm 32 and they are turned back to the collector screen 30 by thenegative potential of the film. However, the positive field of the metallm 24, which is ysubstantially 8,000 volts positive with respect toviewing gun cathode potential, will tend to extend through theinterstices of the coated mesh 28 and draw electrons of beam 44 to thefluorescent screen 26. By adjusting the potential of screen 28 sucientlynegative, it is possible to just prevent any electrons of beam 44 from`penetrating through screen 28.

The beam 38 of the writing gun is then turned on and scanned over thesurface of the insulating film 32 simultaneously while the beam is beingmodulated `by incoming signals applied to .the control grid 35 by theinput circuit 43. The writing beam 38 strikes the insulating coating ataround 3,000 volts which is between the first and the second crossoverpoints on the secondary emission curve of the silica lm 32. In thismanner, beam 38 initiates secondary emission from the silicasurfacegreater than the number of electrons striking the surface frombeam 38, and in those areas, where the beam strikes, the silica surfaceis driven positively by the secondary emission from its potential `of a-20 volts toward the` viewing gun cathode potential or ground. However,no point of the surface of lm 32 can be driven by beam 38 morepositively than ground as low velocity electrons of beam 44 will land onthe lm at that point and drive it back to ground potential or slightlybelow. In those areas of the silica film 32 which are driven positivelyby beam 38 toward the viewing gun cathode potential, the positive fieldof the uorescent screen 24-26 will now penetrate and draw the lowvelocity electrons of beam 44 through the screen 28. These electrons ofbeam 44 which pass through mesh 28, are highly accelerated by theintense positive field between screen 28 and film 24 and will strike theuorescent screen 26 at high velocities to cause luminescence of thescreen. This luminescence will only appear from portions of screen 26adjacent those areas of the storage screen 28 struck by the writing beam38. Where beam 38 does not strike lm 32, the lm remains negative and thepositive eld of the uorescent screen can not penetrate through screen 28and draw beam electrons 44 through to the fluorescent target surface.

Thisvtype of writing provides a picture on the fluorescent screen, whichis dark in the unwritten areas and light or bright in the written areas.The background of the picture will be black and the written areas willbe white. Theoretically, once a signal has been Written on the chargestorage screen 28, it may be seen indefinitely on the luminescent screen26 since with the mode of tube operation as described above, the chargedareas are never discharged by the low velocity beam 44. Actually,however, the stored charge on film 32 .is gradually dissipated byspurious ions in the tube. Y

The stored signal on the insulating film 32 may be erased and thesurface of the film rewritten on. Erasing may be done as described aboveby .shifting the potential of mesh screen 28 in a positive directionuntil the electrons of the spray beam 44 strike film 32 with energiesbetween the first and second crossover points of the secondary emissioncurve of the insulating material. At this point, .all of the surface ofthe .insulating film 32 is driven positively toward the potential of thecollector screen 30. Then, the screen 28 is reset to its negativepotential and a new signal may be written down on the surface of thestorage screen, in the mannerdescribed.

The tube shown in Figure 1 may also be operated in a manner to produce ablack writing on a white background. This may be done by first preparingthe target surface for writing, as described above, by setting screen 28positively to erase any previously written signal or any unequal chargedistribution on the surface of film 32. Screen 28 then is reset toanegative potential inthe order of `two volts below the viewing guncathode potential or ground. The spray beam 44 is then turned on and thevoltage of screen 28 is adjusted yto a point where ythe positive fieldof the fluorescent screen 26 penetrates the openings through mesh 28 todraw the low velocityr electrons through mesh 28 onto the fluorescentscreen. This is indicated, in the absence of the writing vbeam 38, by awhite raster on the fluorescent screen 26. The writing beam is thenturned on and the cathode 19 of the writing beam 18 is set at apotential so that beam 38 will strike the silica film 32 at either belowthe first crossover point or above the second crossover point of thesecondary emission ratio curve for .the material of vfilm 42. Theelectrons of beam 38, at either of these velocities, will initiate asecondary emission which is less .thanunity and `thus drive the surfaceof film 32 negatively. vIf the beam 38 is modulated by incoming signalsfrom source 43 simultaneously While it is being scanned across thesurface of film 32, there will be established on the surface of theinsulating film a negative charge pattern. In thoseareas of the film 32struck by beam 38, the negative Acharge established will be sufficientto neutralize the positive field of the fluorescent screen r26 whichpenetrates through the screen 28. Thus in those areas Vof film A32charged negatively by beam 38, the low velocity electrons of beam 44will not see the positive field of zthe fluorescent screen but will bereflected and turned `back-to the collecting screen 30. In this mannerythen the writing beam 38 will effect a black signal pattern on vthefluorescent screen 26 since no electrons of beam 44 Will penetratethrough screen 28 in the Written areas. I

Also in accordance with the invention, I haveprovided an electron lensor focusing raster, in amanner that Ythe viewing beam electrons passingthrough thestorage screen r 28 are brought to points of focus onthe'uorescent screen. This provides an optimum resolution of the pictureformed on the fluorescent screen. Without the provision of a focusingsystem, the electrons passing through storage grid 28 will becomediffused, and the areas in which they strike, will overlap to providepoor resolution in the visual picture formed on fluorescent screen 26.By providing appropriate spacing between fiuorescent screen 26 and grid28, as well as appropriate fieldstrength on both sides of screen 28,there is formed an electron lens adjacent each aperture of screen 28which focuses the electrons passing through the aperture to a limitedspot on fluorescent screen 26.

Fig. 3 shows a partial enlarged view of the region between screens 28and 26 of the target structure 20 of Fig. 1. As shown in Fig. 3, thespray beam 44 passing through screen 28 is converged to focal points 45,for example, lying substantially in a common plane. The electrons ofbeams 44 passing through these focal points diverge and strike .thetiuorescentfilm 26 `in small Yareas ordots47. It is found in accordancewith the invention that by varying several factors, it is possible tocontrol the size of the luminescent dots 47 and to maintain the sprayelectrons 44 as tiny narrow beams as they emerge through the aperture28. The conditions at which dots 47 are just tangent to one another maybe consideredthe optimum focusing condition.

To provide the optimum focussing conditions for a bright viewing tube,and conveniently low viewing gun voltage, it is essential that the fieldgradient between the focussing mesh 28 and the fluorescent screen 26 becon' siderably greater than the field gradient between the collectorscreen 30 and the focussing grid 28. For example, `in the tube describedin Fig. 1 the difference of potential, Aduring tube operation, betweencollector screen 3ft and the focussing vgrid y28vis in order of 1,000volts. The-distance between these two screens is approximately l cm., sothat the field gradient between the screensy 30 and 28 is substantially1,000 volts per cm. On theother hand, the potential difference betweenthe focussing grid 28 .and fluorescent screen 26 is in the order of8,000 volts, and the distance between screens 28 and 26 in the order of0.3 cm. This space provides then a field between screens 28 and 26 inthe order of 25,000 volts per cm. The ratio of the high potentialgradient between screens 28 and 26, and low potential gradient betweenscreens 30 and 28 is in this case 25,00021000 or 25. Lower fieldgradient ratios are applicable, if brightness or size of viewing beampower supplies are ynot essential.

`Furthermore, it has been shown that the individual beams emergingthrough the focussing grid -28 remain distinctly separated from eachother within a certain v'range of potentials of -screen 28, near thecathode potential of the viewing beam gun. It has also been found ythatif the focussing screen 28 is near the potential of lwhen screen 28 isnegative, the beams are brought respectively to crossover points45 justbeyond the focussing screen 28. The electrons, however, leavingcrossover points 45 do not become Wider at the fluorescent screen 26,but also become narrower with increasing negative potential of thefocussing screen '28. The reason for this effect is that the action ofthe negative field around each Vaperture of screen 28 cuts off theoutside orfringe lsectrons of each electronbundle passing through screenA positive screen 28 is an advantage, if the tube'of the type describedin Fig. 1 is to be'used merely asnonstoring, post acceleration kinescopeimage intensifier.

For the given spacing of 0.3 cm. between screens 28 and 26 and for thegiven field strength ratio of 25, dots 47 formed by beams 44 becometangent to each other. However, if screen 26 were spaced farther fromscreen 28, with the same field strength ratio, spots 47 would overlap,and the resolution of the picture would suffer seriously. "Spacingscreen 26 closer to screen 28, with the same field strength conditions,would not deteriorate picture resolution. But, a closer spacing ofscreens V26 and 28 is undesirable due to the possibility of voltagebreakdown caused by the shorter insulating path between the two screens.If brighter pictures are desired by increasing the potential of screen26, it would be advisable for better insulation between the screens tospace screens 28 and 26 at greater distances at the same time. However,for optimum focussing conditions, it would be necessary to use amorer'negative potential on the focussing screen 28, or it would vbe'even possible to .go to a 'posianzidetto "tive potential for grid 28 ifbeam modulation and storage is not a consideration as mentioned above.

A condition `for optimum beam focus has been described above as that inwhich the field strength ratio is l5. This is not limiting as thefieldstrength ratio is not 'critical and may be varied between 2 and 50, foreX- ample. For any given value of the ratio of the field strengths,there is a range of potentials at which the focussing grid 28 may be setwith a corresponding change in the potential of screen 26 to provide aconditionof optimum focussing. It is only necessary, as described above,to adjust the voltages of screens 28 and 26 for the given spacing ofscreens 28 and 26, to aect the condition of tangent spot condition. Ithas furthermore been found that the aperture opening in screen 28 isalso a determining factor for producing optimum beam focus. Largerapertures in screen 28 require a correspondingly more negative potentialon the screen 28 during tube operation for the same potential on screen26, or in other words, optimum beam focus is produced with larger holediameters in screen 28 by using a smaller potential on screen 28.

Accordingly, as fully described above, and in accordance with theinvention, it is possible to provide a picture beam tube in which thevisual picture is greatly intensified and is also one of highresolution. The result is produced, as described above, by controllingthe electrons falling upon the fluorescent screen by a focussing grid ora lens raster which not only modulates the electrons passing through,but also directs them in well defined beams on to the fluorescent screento provide a high luminescent picture of good definition. Furthermore,it is possible to provide a luminous picture for a period of time by thestorage of signal charges on the control grid or focussing lens rasterwhereby certain information may be retained visibly to an observer for adesired period of time.

Figure 4 discloses a type of image tube using an electron lens rastersystem in accordance with my invention. The tube of Figure 4 consists ofan evacuated envelope 50 having a fluorescent target electrode 52mounted at one end of a metal cylinder 54. The target electrode 52consists of a supporting sheet of glass 56, a fluorescent film 58 havingon its exposedsurface a thin film 60 of conductive material, such as anelectron permeable film of aluminum. On the end of the tube envelope isa photocathode coating electrode 62 formed on a conductive transparentfilm 61. Spaced between the photocathode and target electrode 52 are acollector screen 64 and a storage screen 66, forming an electron lensraster system. On the surface of screen 66 facing photocathode 62 is acoating 67 of insulating material such as silica or magnesium fluoride,for example, and having its secnd crossover potential ofthe secondaryemission curve somewhat below 10,000 volts, in this example. `The meshscreens 64 and 66 correspond in function respectively to screens 30 and28 of Figure l. The aluminum coating 60 of the fluorescent screen isgrounded as is shownQwhile screens 64 and 66 are connected by leads toterminals 68 and 70, respectively. Terminals 68 and 70 can be connectedby a movable switching device 72 respectively to several terminals 1, 2,3 and 1', 2' and 3, which in turn are connected respectively by leads tosources of different potential supplied by a voltage divider 7S, forexample. l

The operation of the device of Figure 4 is that in which the switchingdevice 72 is first positioned so as to connect terminals 70 and 68 toterminals 1 and 1 respectively. This is the position for erasure of thetarget in preparation for signal storage thereon. Screen 66, which maybe considered a backplate of film 67, and screen 64 are now connected toa voltage of minus 9,000 volts, relative to ground. Conductivefilm 61 isconnected to a minus 9,400 volts, relative to ground, during tubeoperation. The photocathode coating 62 is now flooded with light fromclosely positioned light sources 74.` Photoelectrons from thephotocathode coating. 62 pass through the accelerator grid 64 withenergies of around 400 volts and strike the insulating surface 67 ofgrid 66 above rst crossover potential of thesecondary emission curve forthe material of lm 67. The insulating surface 67 is driven to the,potential of screen 64 or substantially a minus,9,000 volts, since thesecondary electrons from coating67 are driven back to coating 67 bythenegative collecting field of screen 66. This procedure erases any chargeon insulating ,coating 67 and establishes a uniform charge over all ofcoating 67 near to the potential of screen 66.

The floodlights 74 are turned olf, and the switch 72 is moved toterminals 2 and 2. This sets screen 66 at a potential of a minus 6,000volts relative to ground, and screen 64 at a potential of a minus94l0volts relative to ground or some 10 volts negative to photocathodepotential. This produces a high accelerating field of some 3,400 voltsbetween screens 34 and 36. This accelerating field will penetratethrough the interstices of negative screen 64 and will accelerate anyelectrons leaving photocathode 62 through screen 64 onto the insulatingcoating 67. Furthermore, the penetrating of the accelerating fieldthrough the negativescreen 64 produces an electron lens fielddistribution at each interstice of the screen 64. All electrons thenpassing through each interstice of screen 64 are converged and broughtto a focus at well defined spots on the insulating coating 67.

If now an image of an object to be viewed is focused by any appropriatelens system, indicated by 76, upon the photocathode 62, photoelectronswill be emitted from each portionof the photocathode 62 in numberscorresponding to the amount of light falling on that portion. Thesephotoelectrons penetrate through negative screen 64 and are focused oncoating 67, as described, by lens fields formed in each interstice ofscreen 64. The photoelectrons will strike the dielectric, coating 67with energies around 3,400 volts and .will initiate a secondaryemission, which is repelled by the negatively charged screen 64.However, the intensely positive 'field of some 6,000 volts ybetweenstorage grid 66 and aluminum film 60 penetrates through the apertures otstorage screen 66 and draws the secondary emission from coating 67through and onto the fluorescent screen 58. Also, in the mannerdescribed above for the operation of screen 28 of Figure 1, storage grid66 provides a lens raster so that the secondary electrous passingthrough the apertures of screen 66 are focused to well-defined dots onthe luminescent screen 58. Thus, there is established on the fluorescentscreen a visual picture which corresponds to the optical image focusedupon the photocathode 62 by the lens system 76.

The preferred operation of the modificationof Figure 4 is to flash theoptical picture` or image on the photocathode 62 for a very short timeand thus write or store such an image electrically on grid 66. Theelectrically stored image can then be reproduced visually on thefluorescent screen 60 for any desired length of time to provide astorage viewing of the picture. This is done by first erasing any chargepattern on the storage screen 66 in the manner described above. Lights74 are turned off and switch 72 is moved to position 2--2 for writing" acharge image on the storage` screen. A picture is optically projected onthe photocathode 62 and a corresponding charge pattern is established oncoating 67 in the manner described above. The picture may be flashed onfor only an instant or for longer before being cut off. To view thestored information on coating 67, the switching device 72 is moved nextto terminals 3 and 3 respectively. This connects accelerator screen 64to a potential of minus 8,800 volts relative to ground which is 600volts positive relative to photocathode potential. The `storage screen66 is also connected to a potential of minus 9,420 volts which is 2Ovolts negative to photocathode potential.` Simultaneously, theinsulating coating 67 is 59 by .grid 66 to substantially the lsamepotential negative to the photocathode potential. The photocathode 62 isnow illuminated by light from sources 74. yA uniform photoemission nowleaves all portions of photoca'thode 62 and is accelerated throughscreen 64 "toward the storage screen 66. In those portions of thevstorage screen which were charged positively during the writingoperation, the positive field from the target 52 will penetrate throughscreen 66 and draw the photoelectrons through to the luminescent screenin amounts proportional to the positive charge established in each areaof coating 67. However, in those areas of coating l67 which wereuncharged during the Writing operation, '.fphotoelectrons will berepelled back 'to the collector .screen 64 by the negative potential ofcoating 67. The Iphotoelectrons, which do penetrate Vthrough screen 66rWill-be focused onto the fluorescent layer, in the manner describedabove for the tube of Figure 1, and will rproduce :a visual picture ordisplay in accordance to the distribution of positive chargesestablished on the insulated surface 67 of the storage screen 66.Furthermore, this display will remain for a period of time as thepotential of screen 66 is adjusted until all portions of the storagescreen 66 are negative to photocathode potential and no :photoelectronsland on the target to discharge the charge pattern. In this manner,there is thus provided a stored .picture on the fluorescent screen whichlis much brighter `than the directly viewed picture. Due to the focusingaction of the storage grid, the viewed picture is of good definition.Resolutions up to 200 limit per linear inch of the luminescent screenare easy to obtain.

The optical picture may be pulsed or gated and the .pattern produced canbe'observed between pulses. Such an application would be of value inobserving stroboyrscopically rapid periodic motions. Also, suchapplications would be of value in X-ray observations where the patientcannot be safely exposed to X-rays for long periods of time.

The tube of Figure 4 has been described as operating with a positivecharge pattern on storage coating 67. However, the potential of screen66 may be set during writing so that the photoelectrons released by theoptical picture will strike coating 67 with energies above secondcrossover potential to produce in the light areas a negative chargepattern. This will produce on the fluorescent screen 58 a negativepicture showing luminescence in the dark areas.

The image tube of Figure 4 depends for picture definition upon the lensraster which provides focusing of the electron passing through eachaperture of screen 66 upon thefluorescent screen 58. The requisites forproper focusing conditions are similar to those described for the tubeof Figure 1. For example, the tube of Figure 4 provides a-low potentialgradient between the accelerating screen 64 and screen 66 and arelatively high Vpotential gradient between screen 66 and thefluorescent screen.

.The potentials described at which the tube of Figure 4 is operated areillustrative and knot meant to be limiting. For example, each of theelectrodes of the tube of Figure 4 may be operated at half the values ofthe respective potentials set forth above, to produce similar results.

The tubes of Figures l and 4 have been described as storage devices inwhich the focusing screens respectively y28 and 66 are coated with adielectric lllmfor .the storage ofcharges. My invention is not limitedto storage tubes, but also includes viewing tubesin which screenelectrodes ZSand 66 are-not coated and provide merely focusing of theyelectrons `on the fluorescent screen. VSuch beam intensifier tubes may,in the case of Figure l have only one (scanning) gun, and represent apost deflection acceleration oscilloscope with excellent contrastbecause no landing of secondary electrons stemming .from theacfcelerati'on assembly at the luminescent screen takes place. Intheicase of Figure 4, Ian image vintensifier 'for larger -ytures can bemade which is4V relatively short and may Abe Agated by applying lowvoltage electrical pulses to the `electron lens raster grid near the'luminescent screen.

What I claim is:

l. An electron discharge device comprising an electron source, afluorescent screen spaced fromr said electron source, a mesh screenelectrode positioned between said electron source and said fluorescentscreen and closely spaced. from said fluorescent screen, and a secondmesh screen electrode positioned between said first mesh screen and saidfluorescent screen, said second mesh screen electrode having aninsulating coating on the surface thereof facing said first mesh screenelectrode for storing a charge pattern, means establishing a firstpotential gradient between said frst and second mesh screens and ahigher potential gradient relative to said first potential gradientbetween said second mesh screen and said fluorescent screen wherebyelectrons from said source and passing through said screens are focusedon said fluorescent screen.

2. An electron discharge device comprising, an electron source, afluorescent screen spaced from said electron source, a mesh screenelectrode positioned between said electron source and said fluorescentscreen and closely spaced from said fluorescent screen, and a secondmesh screen electrode positioned between said first mesh screenelectrode and said fluorescent screen, said second mesh screen electrodehaving an insulating coating on the surface thereof facing said firstmesh screen electrode for storing a charge pattern, means establishing afirst potential gradient between said first and second mesh screens anda higher potential gradient relative to said first potential gradientbetween said second mesh screen electrode and said iluorescent screen,said means including potential sources respectively connected to saidelectron source and said second mesh for maintaining said second meshscreen and said electron source respectively at potentials differingonly in the range from 0-100 volts.

3. An electron discharge vdevice comprising, an electron source, afluorescent screen spaced from said electron source, a mesh screenelectrode positioned between said electron source and said fluorescentscreen and closely spaced from said fluorescent screen, and a secondmesh electrode screen positioned between vsaid first mesh electrodescreen andsaid fluorescent screen, said second mesh screen electrodehaving an insulating coating on the surface thereof facing said firstmesh screen electrode for storing a charge pattern, means establishing afirst potential gradient between said first and second mesh screens anda high potential gradient relative to said first potential gradientbetween-said second mesh screen and said fluo- .rescent screen, saidmeans including potential sources respectively connected to saidelectron source and said second mesh screen for maintaining said secondmesh screen, respectively at a few volts negative relative to thepotential of said electron source.

4. An electron discharge device comprising, a target assembly includinga fluorescent screen, a focussing screen electrode and collector screenelectrode, said screen electrodes mounted-in parallel spaced relation toa surface of said fluorescent screen', said second mesh screen electrodehaving an insulating coating on the surface thereof facing said vfirst'mesh-screen electrode for storing a charge pattern, and means `forproviding a flow of electrons through said screen electrodes onto saidfluorescent screen.

5. An electron discharge device comprising, a target assembly includinga fluorescent screen, a focussing screen electrode and collector screenelectrode, said screen electrodesl mounted in parallel spaced relationto a surface of said fluorescent screen with said focussing `screenelectrode between said collector screen electrode and said fluorescentscreen, an insulating film on the surface of said focussing screenelectrode facing said collector electrode, and means for providing amodulated stream of electrons through said collector-screen' onto saidinsulatl1 ing film, and means for providing a second'stream of electronsthrough said screen electrodes onto said fluorescent screen.

6. An electron storage device comprising, a fluorescent screen, acollector screen spaced substantially parallel to one surface of saidfluorescent screen, a focussing screen electrode positioned between saidcollector and fluorsecent screens, said second mesh screen electrodehaving an insulating coating on the surface thereof facing said firstmesh screen electrode for storing a charge pattern, means forestablishing a potential gradient on both sides of said focussing screenwith the ratio of the field strength between said focussing andfluorescent screens to the field strength between said focussing andcollector screens being in the rang from 2 to 50, and means includingsaid focussing screen for providing a modulated flow of electronsthrough said focussing screen onto said fluorescent screen.

7. An electron storage device comprising, a fluorescent screen', acollector screen spaced substantially parallel to one surface of saidfluorescent screen, a focussing screen electrode positioned between saidcollector and fluorescent screens said second mesh screen electrodehaving an insulating coating on the surface thereof facin'g said firstmesh screen electrode for storing a charge pattern, means forestablishing a potential gradient on both sides of said focussing screenwith the ratio of the field strength between said focussing andfluorescent screens to the field strength between said focussing andcollector screens being in the range from to 30, and means for providinga flow of electrons through said focussing screen onto said fluorescentscreen.

8. An electron storage device comprising, a fluorescent screen, acollector screen spaced substantially parallel to one surface of saidfluorescent screen, a focussing screen electrode positioned between saidcollector and fluorescent screens, said secon'd mesh screen electrodehaving an insulating coating on the surface thereof facing said firstmesh screen electrode for storing a charge pattern, means forestablishing a potential gradient on both sides of said focussingscreen' with the ratio of the field strength between said focussing andsaid fluorescent screens to the field strength between said focussingand collector screens being equal to 25, and means including saidfocussing screen for providing a modulated stream of electrons throughsaid focussing screen onto said fluorescent screen.

9. An electron storage device comprising, a fluorescent screen, acollector screen spaced substantially parallel to one surface of saidfluorescent screen, a focussing screen electrode positioned between saidcollector and fluorescent screens, said focussing screen having aplurality of uniform apertures therethrough said second mesh screenelectrode having an insulating coating on the surface thereof facingsaid first mesh screen electrode for storing a charge pattern, meansforestablishing a potential gradient on both sides of said focussingscreen with the ratio of the field strength between said focussing andfluorescent screens to the field strength between' said focussing andcollector screens being in the range of 10 to 30, and means includingsaid focussing screen electrode for providing a'modulated flow ofelectrons through said focussin'g screen and said fluorescent screen.

l0. An electron discharge device comprising, a target assembly includinga fluorescent screen, a storage grid electrode and a collector screenelectrode, said electrodes being mounted in parallel spaced relation toa surface of said fluorescent screen with said storage grid electrodebetween said fluorescent screen an'd said collector screen, said secondmesh screen electrode having an insulating coating on the surfacethereof facing said first mesh screen electrode for storing a chargepattern, means including an electron source for providing low velocityelectrons directed toward said target assembly and means 12 including asecond electron source for providing high velocity electrons directed atsaid target assembly.

1l. An electron discharge device comprising, a target assembly includinga fluorescent screen, a storage `grid electrode and a collector screen'electrode, said electrodes mounted in parallel spaced relation to asurface of said fluorescent screen with said storage grid electrodebetween said fluorescent screen and said collector screen, a firstelectron gun for providing a low velocity electron beam along a pathintercepting said target assembly, an'd a second electron gun forproviding a high velocity electron beam directed at said targetassembly.

12. An electron discharge device comprising, a target assembly includinga fluorescent screen, a storage grid electrode and a collector screenelectrode, said electrodes mounted in parallel spaced relation to asurface of `said fluorescent screen with said storage grid electrodebetween said fluorescent screen and said collector screen, a rstelectron gun for providing a low velocity electron beam along a pathintercepting said target assembly, and a second electron gun forproviding a high velocity electron beam directed at said targetassembly, and an insulating coating on the surface of said storage gridelectrode facing away from said fluorescent screen surface.

13. An electron discharge device comprising, a target assembly includinga fluorescent screen', a storage screen electrode and a collector screenelectrode, saidelectrodes mounted in parallel spaced relation to asurface of said fluorescent screen with said `storage screen electrodebetween said fluorescent screen and said collector screen, a firstelectron gun for providing low `velocity electrons directed at saidtarget assembly, and a second electron gun for providing a high velocityelectron beam along a path intercepting` said target assembly, and aninsulating coating on the surface of said storage screen electrodefacing away from said fluorescent screen surface, means for establishinga potential gradient on both sides of said ystorage screen electrodewith the ratio of the field strength between said storage screen andfluorescent screen to the field *strength between said storage screenand collector screen being in the range from 2 to 50, and means forproviding a modulated stream of electrons through said storage screenelectrode onto said fluorescent screen.

14. An electron discharge device comprising, a target assembly includinga fluorescent screen, a storage mesh screen electrode and a collectormesh screen' electrode,

said electrodes mounted in parallel spaced relationto a surface of saidfluorescent screen with said storage' screen electrode between saidfluorescent screen and said4 collector screen, a first electron gunincludingan electron' source for providing low velocity electronsdirected at -said target assembly, and a second electron gun forproviding a high velocity electron beam directed at said targetassembly, and an insulating coatin'g on the surface of said storagescreen electrode facing away from said fluorescent screen surface, meansestablishing a first potential gradient between said collector screenand storage `screen and a high potential gradient relative to said firstpotential gradient between said storage screen and said fluorescentscreen, said means including potential sources respectively connected tosaid electron source and said storage screen' for maintaining saidelectron source and said storage screen respectively at potentialsdiffering only in the range of zero to volts.

15. An electron discharge device comprising, a target Yassemblyincluding a fluorescent screen, a plurality of screen electrodes mountedin parallel spaced relation to a surface of said fluorescent screen',photoemissive means for providing a modulated stream of electronsthrough said screen electrodes onto said fluorescent screen,` and meansfor connecting said fluorescent screen,` said photoemissive means andsaid electrodes to sources of potential to provide a high potentialgradient between xsaid fluorescent screen and the adjacent one of saidscreen electrodes and a high potential gradient between said screenelectrodes whereby electrons passing through said screen electrodes arefocused on said adjacent screen electrode and saiduorescent screen.

16. An electron discharge device comprising, a targetl assemblyincluding a fluorescent screen, a focussing screen electrode and acollector screen electrode, said screen electrodes mounted in parallelspaced relation to a surface of said fluorescent screen, .an insulatingcoating on said focussing screen electrode, photoemissive means vforproviding a stream of electrons through said collector and focussingscreen.

17. The method of operating an electron discharge device consisting of auorescent screen an'd a mesh screen electrode mounted in parallel spacedrelationship 18. The method of operating an electron dischargev deviceconsisting of a iluorescent screen and a mesh screen electrode mountedin parallel spaced relationship to one surface of said fluorescentscreen, said method comprising the steps of, establishing anelectrostatic potential eld gradient on both sides of said mesh screenelectrode with the ratio of the eld strength between the lfluorescentscreen and said mesh screen electrode to the ield strength on' theopposite side of said mesh screen electrode being in `the range from 10to 30, and providing a modulated electron ow through said mesh electrodeonto said fluorescent screen.

References Cited in the ile of this patent UNITED STATES PATENTS2,107,782 Farnsworth et al. Feb. 8, 1938 2,254,140 Farnsworth Aug. 26,1941 2,258,294 Lubszynski et a1. Oct. 7, 1941 2,280,191 HergenrotherApr. 21, 1942 2,404,077 Leverenz July 16, 1946 2,584,814 Rosenberg et alFeb. 5, 1952 2,619,608 Rajchman Nov. 25, 1952 2,686,219 Linden'blad Aug.l0, 1954

